Spatial and temporal genetic control is necessary to better dissect the role of specific genes and cell populations in development, disease, and therapy. Site-specific recombination mediated by Cre recombinase is a powerful genetic tool to manipulate genetic elements in model organisms (Nagy, Genesis 26:99, 2000). When placed under the control of a tissue-specific promoter, Cre/LoxP-mediated recombination allows tissue-specific investigation of gene functions (Nagy, Genesis 26:99, 2000). Fusing Cre with a mutant form of the estrogen receptor (ER) ligand binding domain further enables temporal control of recombination (Nagy, Genesis 26:99, 2000). While this conditional CreER system allows for temporal control, the spatial control through tissue-specific promoters is limited by relatively broad activation in all target cells, non-specificity of many of such cell type-‘specific’ promoters, and/or the lack of validated promoters in certain cell types.